Magnetic ordering and spin structure in Ca-bearing clinopyroxenes CaM2+(Si, Ge)2O6, M=Fe, Ni, Co, Mn

The compounds CaFeSi2O6 (hedenbergite), CaNiGe2O6, CaCoGe2O6 and CaMnGe2O6 have been synthesized by hydrothermal or ceramic sintering techniques and were subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/c, Z=4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite M1 chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a–c plane and form an angle of 43° with the crystallographic a-axis. Magnetic ordering in CaFeSi2O6 causes significant discontinuities in lattice parameters, Fe–O bond lengths and interatomic Fe–Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe2O6 and CaNiGe2O6 show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM coupling within and an AFM coupling of spins between the M1 chains. The moments lie within the a–c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi2O6 (+40.2 K) to CaCoGe2O6 (+20.1 K) and CaNiGe2O6 (−13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the M1 chains. CaMnGe2O6 finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a–c plane.